Thermoplastic films are widely used in personal care items, for example, as the outer layer of a diaper or other disposable personal hygiene product. For a variety of reasons, including cost, comfort, conservation of resources and minimizing waste, it is desirable to have as thin a film as possible while maintaining other necessary properties of the film.
Desirable qualities of thermoplastic films include being liquid impervious, vapor permeable (e.g., breathable), bondable to other layers of the personal care item, and having sufficient physical strength to be processed into a finished article. Strength is an important consideration when using thermoplastic films for packaging, for example, as an outer packaging for consumer goods. Breathable films having a sufficient strength and basis weight may be particularly useful as packaging for products that need to release odors resulting from the manufacturing process.
Thermoplastic films may be formed by extruding a molten polymeric composition onto a chill roller, where it is immediately cooled to make a solid film Processing of the film includes a variety of steps, including heating, cooling and stretching to produce a final film product having a thickness of 72 times or less than the initial thickness. Stretching in the machine direction (MD) forms a highly oriented thin gauge film, which is referred to as machine direction orientation (MDO). MDO may be useful, however, may also result in qualities such as reduced cross-directional (CD) tensile strength, impact strength, tear strength and slow puncture resistance, particularly in thinner films.
Current methods for making thin gauge thermoplastic films include those described in U.S. Pat. No. 7,442,332 (Cancio et al). In this process, a large part of the stretching (over half) of the web occurs between the extruder die and a first nip (i.e., in the “melt curtain”). In such a cast process, two drawbacks are the phenomenon known as “draw resonance,” which results in uneven film thickness, and the formation of holes in the film. These problems increase with production speed, and furthermore may limit the types of polymeric compositions that can be used. Overcoming these issues requires slowing production, which ultimately results in increased cost.
A need exists, therefore, for thin thermoplastic films, which have limited MDO and desirable properties such as lack of holes, good breathability, good tensile strength and tear strength properties, and which can be produced economically and efficiently on high-speed production lines.